Powder mixture for making an undercoating agent for steel surfaces

ABSTRACT

The invention concerns a zinc-containing corrosion-protecting undercoating agent for steel surfaces. The undercoating agent, which contains no organic solvents, consists of a mixture of dry, water-emulsifiable dispersion agent in powder form and water-reactive zinc in powder or flake form which are mixed with water immediately before treatment. The corrosion-protection properties of undercoating agents prepared in this way are as good as those of prior art single-component zinc-dust undercoating agents using only organic solvents.

DESCRIPTION

The invention relates to a powder mixture for making an anticorrosivezinc-containing undercoating agent for steel surfaces.

It is known to use zinc dust and/or zinc flakes as acorrosion-inhibiting pigment in organic undercoating agents. Theundercoating agents consist essentially of a synthetic resin, which isdissolved in an organic solvent to produce a soft consistency duringuse. Aromatic hydrocarbons, like xylol, are primarily considered asorganic solvents. The undercoating agent is stored in its liquid form inclosed containers and is delivered in this manner. For a specified useit is directly applied to the previously cleaned steel surface. Byevaporating the solvent, the mixture hardens and forms a solid closedlayer in which the zinc dust is contained in a finely distributed form.When moist air penetrates into the undercoating, mainly basic zinccarbonate (4ZnO·CO₂ ·4H₂ O) forms on the zinc flakes, which resultsduring an increase in volume and an increase in the density of theundercoating and protects the steel surface lying therebelow fromdestruction. A further anticorrosive effect is due to the cathodicprotective action of the zinc dust pigment.

Considerable amounts of organic solvents are emitted into thesurrounding air during the application of the known zinc dustundercoating agents. The thus caused environmental damage has beenaccepted up to now because other binding agents, for example on a waterbase, did not appear to be suited because of the, to be expected,chemical reaction with the zinc powder.

Starting out from this, the basic purpose of the invention is to providea one-component powder mixture to make an undercoating agent which canbe environmentally friendly produced, stored and applied, and which hasstill good corrosion-inhibiting characteristics.

To attain this purpose the invention suggests to first produce a mixtureof a dried, water-reemulsifiable powdery dispersion agent and awater-reactive zinc in a powder or flake form and which can be storedand transported in water-tight bags.

The mixture contains advantageously a water-reemulsifiable dispersionagent from the following groups:

a vinyl acetate copolymer, in particular, a vinyl acetate-ethylenecopolymer, a vinyl acetate-vinyl ester copolymer or a vinylacetate-vinyl laurate-vinyl-versatate copolymer, an acrylate, anethylene-vinyl laurate-vinyl chloride copolymer, a styrene copolymer, inparticular a styrene acrylate or a styrene butadiene.

The mixture has furthermore advantageously a powdery filler or extenderfrom the talcum, barium sulfate and mica group and/or a preferablyorganic pigment, which filler or extender is actually known in thelacquer industry. These materials help to improve thecorrosion-inhibiting characteristics and to adjust the desired rheology.

In order to obtain during the application an easily homogenizablemixture out of the powdery parts with water, the use of actually knownadditives, like wetting agents, defoamers, thixotropic agents, agentspromoting the film formation, is of an advantage.

The dry mixture of the invention contains advantageously the followingparts:

1 to 30 percentage by weight of dispersion powder from the group:

vinylacetate-copolymer, in particular vinyl-acetate-vinylester-copolymeror vinyl acetatevinyllaurate-vinyl-versatate-copolymer acrylate

ethylene-vinyllaurate-vinylchloride-copolymer

styrene-copolymer, in particular styreneacrylate, styrenebutadiene

70 to 99 percentage by weight of pigments/fillers consisting of:

2 to 100% of zinc powder and/or zinc flakes

0 to 98% of fillers like talcum (3MgO·4SiO₂ ·H₂ O), barium sulfate(BASO₄) or mica (K₂ O·3Al₂ O₃ ·6SiO₂ ·2H₂ O, or "PLASTORIT" types)

the remainder: powdery additives, like defoamers, wetting agents, agentspromoting film formation, thixotropic agents.

The powdery dry mixture can be stored and transported in water-proofbags. Shortly before the application, 100 parts by weight of the powderymixture is mixed by agitation with 5 to 50 parts by weight of water.After the water preferably 20 to 30 parts by weight of water, to form aviscous coating compound has evaporated, a closed continuous coating isformed. The good corrosion-inhibiting characteristics of the coatingproduced in this manner permit one to conclude that the zinc powder,which is reactive with water, is extensively shielded against thereaction with water by the water-absorbing dispersion agent during theshort application time.

Exemplary embodiment 1

25 parts by weight of a powdery vinylacetate-vinyl-ester-copolymer asdispersion powder were mixed with 48 parts by weight of zinc powder, 25parts by weight of talcum and a remainder of powdery additives forming adry powdery mixture.

Exemplary embodiment 2

10 parts by weight of styreneacrylate as dispersion powder, 64 parts byweight of zinc flakes, 25 parts by weight of barium sulfate fillers anda remainder of powdery additive were mixed forming a dry, powderymixture.

100 parts by weight of the dry, powdery mixtures according to theexemplary embodiments 1 and 2 were mixed by agitation with 30 parts byweight of water and were immediately thereafter applied to a cleanedsteel surface as an undercoating.

After one week of drying time at a temperature of 23° C. and 50%humidity, the coated surface was subjected to the salt spray testaccording to DIN 50021-SS. Rusty spots, as they otherwise occur withwater-based undercoating agents, were not found.

In summary, the following can be stated: The invention relates to ananticorrosive zinc-containing undercoating agent for steel surfaceswhich is free of organic solvents and consists of a mixture of a dried,water-reemulsifiable powdery dispersion agent and a water-reactive zincin powder or flake form which is mixed with water immediately beforetreatment. The undercoating agent produced in this manner hascorrosion-inhibiting characteristics which are just as good as the knownsingle-component zinc-dust undercoating agents exclusively dissolved inorganic solvents.

We claim:
 1. A powder mixture for making an anticorrosivezinc-containing undercoating agent for steel surfaces, comprising a drymixture of a water-reemulsifiable powdery dispersion agent and awater-reactive zinc in powder or flake form said dispersion agent beingselected from the group consisting of a vinyl acetate copolymer, anacrylate, an ethylene-vinyl laurate-vinyl chloride copolymer and astyrene copolymer.
 2. The powder mixture according to claim 1, whereinthe dispersion agent is selected from the group consisting of:a vinylacetate-ethylene copolymer, a vinyl acetate-vinyl ester copolymer, avinyl acetate-vinyl laurate-vinyl versatate copolymer, styrene acrylateand styrene butadiene.
 3. The powder mixture according to claim 1,wherein the mixture contains at least one powdery filler selected fromthe group consisting of talcum, barium sulfate, mica and an organicpigment.
 4. The powder mixture according to claim 1, wherein the mixtureadditionally contains at least one powdery additive selected from thegroup consisting of a defoamer, a wetting agent, a thixotropic agent,and a film formation promoting agent.
 5. The powder mixture according toclaim 1, wherein said dry mixture comprises:1 to 30 percent by weight ofthe dispersion agent and 70 to 99 percent by weight of pigments/fillersselected from the group consisting of 2 to 100% of at least one of zincpowder and zinc flakes, 0 to 98% of a filler selected from the groupconsisting of talcum, barium sulfate and mica and the remainder being atleast one powdery additive selected from the group consisting of adefoamer, a wetting agent, an agent promoting film formation and athixotropic agent.
 6. A method for the manufacture and application of ananticorrosive undercoating agents for steel surfaces comprising thesteps of mixing the powder mixture according to claim 1 with water toform a viscous coating compound, applying the coating compoundimmediately thereafter to a steel surface and drying the compound byevaporating the water.
 7. The method according to claim 6, wherein 5 to50 parts by weight of water are mixed with 100 parts by weight of apowder mixture.
 8. A method of making an anticorrosive undercoatingagent comprising the step of mixing water with the powder mixture ofclaim 1 to form a viscous coating compound.
 9. The method according toclaim 8, wherein 5 to 50 parts by weight of water are mixed with 100parts by weight of the powder mixture.